Recovery of useful materials from wastes is a growing goal in modern society. Landfills are becoming filled to capacity and new sites are hard to find. A second motivation for recovering wastes is the global depletion of raw materials needed to make fresh material. Polymer waste, often made from petroleum products, is a fertile area for recovery solutions. Man-made polymers generally do not degrade quickly and petroleum will eventually be depleted.
Mixed wastes present unique problems for recovery. In mixed wastes, it is necessary to recover the desired material without fouling contamination from other components in the waste. One exemplary mixed waste system involves synthetic fiber production.
Compared with single component filaments, fibers made from two components (bicomponent fibers) have improved properties for some applications. One popular bicomponent fiber has a polycaprolactam sheath and a polyethylene terephthalate core. This type of fiber is especially useful in making non-woven webs since nylon 6 melts at a lower temperature than polyethylene terephthalate allowing, on heating to at least the melting point of nylon 6, spot welding where individual filaments cross.
However, in the preparation of these blends, large quantities of material may be produced which are not suitable for commercial use. Also, used materials are targeted for disposal when useful life is over. As discussed, the ever growing presence of manmade waste in landfills affects the disposal of used materials. So, it is an important commercial and environmental consideration to regenerate and recover the constituents of the blend for reuse. The recovery process, however, to be economically acceptable must return the constituents in high yield and purity without excessive loss through decomposition or side reactions. When applied to blends, known processes for separation and recovery of caprolactam do not produce the desired monomers in high yields and adequate purity.
For example, U.S. Pat. No. 3,317,519 to Lazarus et al. describes a recovery process including heating a mixture of polycaprolactam and polyester with an aqueous alkali metal hydroxide at an elevated pressure and precipitating the thus formed homocyclic aromatic dicarboxylic acid by acidification with a strong acid and recovering caprolactam and glycol from the filtrate. This method, because the depolymerization takes place simultaneously (or nearly simultaneously) with the separation of polymers, produces a caprolactam monomer which is tainted with glycols, especially ethylene glycol. The caprolactam monomer is, therefore, unsuitable for reuse in making nylon 6 for fiber applications.
Similarly, Dmitrieva et al. in "Regeneration of .epsilon.-Caprolactam From Wastes In the Manufacture of Polycaproamide Fibres and Yarns", Fibre Chemistry, March 1986, pp. 229-241, describe a method of recovering monomers from a mixture of polycaprolactam and polyester waste wherein the waste mixture is subjected to hydrolysis in the presence of water. This process, simultaneously depolymerizing and recovering, results in caprolactam tainted with ethylene glycol.
Czechoslovakian Paten Application No. 143502 describes a process for recovering polycaprolactam from mixed waste such as is formed when processing old tires containing polycaprolactam cord. The method is based on first dissolving the polyamide in water or a solvent for the polyamide including acid solvents. The non-polyamide portion is separated by filtration. Where acidic solvents are used, the Czechoslovakian application insists that polycaprolactam must be precipitated prior to depolymerization.
Surprisingly and contrary to the above teachings, it has been discovered that substantially pure caprolactam can be recovered from mixed nylon 6 and other waste by dissolution in acidic solvents without the need for precipitation of the dissolved polycaprolactam prior to depolymerization.